The invention relates to a brake disk, preferably for land vehicles, having axial friction surfaces, particularly an internally ventilated brake disk having two friction rings connected by way of webs, and to a method for its production.
A brake disk of the above-mentioned type is known from German Patent document DE G 298 13 236. The brake disk illustrated in that document is particularly suitable for the high stress occurring in the case of commercial vehicles and was also found to be resistant to wear. For everyday use, a service life of 300,000 km or more in trucks is not unusual. Nevertheless, a basic demand exists for a further increase of the brake disk's resistance to wear while the manufacturing costs of the brake disk should remain economically justifiable.
Although, more recently, the use of ceramic brake disks has been discussed which, in the sports car field, are already being installed in small series, the manufacturing costs of such brake disks are within the range of several thousand Deutsche Mark. Particularly in the field of commercial vehicles, the use of such brake disks therefore seems uneconomical. There is, on the contrary, a demand for a significant increase of the resistance to wear while the manufacturing costs of the brake disk remain justifiable.
However, it is not simply possible to increase the resistance to wear of the brake disk only by increasing the hardness of the brake disk material because the brake disk should not only be resistant to wear but must also meet additional marginal conditions.
By means of the alloyed types of gray cast iron disclosed in German Patent Document DE G 298 13 236, particularly the demands on the heat crack resistance could be improved while the dissipation of heat is simultaneously good and the efficiency is therefore high. In the case of these materials, the wear behavior is also improved in comparison to conventional brake disks made of gray cast iron by the formation of secondary carbides in the structure. These requirements are satisfactorily met for most demands in the vehicle construction by gray cast iron materials. Nevertheless, there is a demand for another significant increase of the wear resistance of the brake disk. However, such an increase of the wear resistance cannot simply be achieved in that the hardness of the gray cast iron is increased by a variation of the alloying additions because other characteristics of the brake disk may deteriorate with an increasing hardness, such as its tendency to form cracks or its thermal conductivity. In the event of an extreme stressing, brake disks with an improved design and selection of material may still have an unacceptable heat crack formation. Another remaining problem with brake disks made of gray cast iron materials is a non-uniformity of the braking torque (“brake judder”), which occurs under certain operating conditions.
Brake disk materials therefore have to meet very different demands, which partly require opposing characteristics for the materials.
Thus, they are used for transmitting the braking torque. For this purpose, they should have a high stability, a high breaking elongation capacity and a high modulus of elasticity.
As a result of the heat occurring during braking, these brake disk materials should also permit high or good heat dissipation. For this purpose, they should have a high thermal conductivity and a high degree of emission.
A high heat crack resistance should also be demanded. For this purpose, the brake disks should have a high thermal conductivity, a low modulus of elasticity and a low thermal expansion.
A low wear of the disk brake is also desirable. For this purpose, a high hardness, a high strength and a high oxidation stability are advantageous.
Frictional characteristics which are as uniform as possible also require a high homogeneity of the material even in the case of high thermal stress, as well as the avoidance of fluctuations in wall thickness as a result of a non-uniform wear.
As in the case of all unsprung masses at the vehicle axle, a weight which is as low as possible, and manufacturing costs which are as low as possible, are also desirable.
The gray cast iron used in the case of the brake disk of the above-mentioned type—particularly with a C-content of from 3.8 to 4%, an Mo-content between 0.6 and 0.8%, and preferably a Cr-content of from 0.2 to 0.3%, as well as preferably other additions, such as Mn, P, S, Cu and/or Ni—is already quite resistant to wear while the manufacturing costs are relatively low.
However, in the field of commercial vehicles, where a disk change may require relatively high expenditures, particularly an increase of the wear resistance to such an extent that, during the service life of the vehicle, a disk change can be eliminated, would be desirable. In this case, the brake disk costs may definitely also increase, but only such that an economic advantage still remains in comparison to the costs of a disk change.
In view of the above problems, the invention is based on the object of providing a brake disk, preferably a gray cast iron brake disk which, while its manufacturing is easy, is as wear-resistant as possible. Preferably, the other characteristics of the brake disk should thereby be impaired as little as possible in comparison to the pertaining state of the art. An advantageous manufacturing method for brake disks according to the invention should also be provided.
The invention achieves this object by providing a brake disk with axial friction surfaces, preferably for land vehicles, particularly an internally ventilated brake disk having two friction rings connected by way of webs. The brake disk is provided on at least one of its axial friction surfaces, in sections or entirely, with a metallic, non-ceramic coating, and a method of making the same.
The invention accordingly provides a brake disk with axial friction surfaces, preferably for land vehicles, which brake disk may also be internally ventilated and is provided at least on one of its axial friction surfaces, in sections or entirely, with a metallic non-ceramic coating.
Since, instead of a ceramic coating, a metallic coating is used, the adherence of the coating is optimized in a simple manner in comparison to brake disks with ceramic coatings. This brake disk is particularly suitable for trucks, but can also be used in passenger cars.
Particularly preferably, the brake disk has a disk-type core made of a cast iron material which, at least on one of its axial outer surfaces, is provided with a metallic injection-molded coating or an explosion coating.
Although coatings which protect against wear on friction surfaces of brake disks are known per se (European Patent Document EP 0 674 114 B1), these coatings on a ceramic bases have favorable wear characteristic but present adhesion problems because of different characteristics compared to the base material of the gray cast iron brake disks and because of the thermal shock stressing that occurs during coating.
In contrast, the invention provides, in a simple manner, a cost-effective coating of brake disks which protects against wear and is compatible with brake disk materials made of cast iron—particularly gray cast iron materials. Here, it is surprisingly possible, while retaining the favorable characteristics of these materials optimized for the use in brake disks, to increase the service life with respect to wear such that the brake disks have to be exchanged even less often; thus, for example, in rarely occurring, particularly wear-intensive applications because of reaching the wear limit.
Furthermore, the coating permits a reduction of the heat crack formation of the brake disk, the effect of the brake judder because of thickness fluctuations of the brake disk arising during the operation also being avoidable.
In the case of the invention, the coatings are preferably applied to the brake body made of gray cast iron by a thermal spray coating method, which brake body thereby forms a type of core.
Although thermal spray coating methods are known per se, thus, for example, when coating piston rings (see, for example, K. Kirner, “Application of Layers to Technical Parts by Means of Thermal Spraying Methods”; from: Kunst et al “Wear-Inhibiting Layers”, Volume 436, Kontakt & Studium, Werkstoffe, Expert Publishers). Their use in the field of coatings of brake disks for producing a metallic brake disk coating, however, has not yet been considered because it was believed that the metallic injection molded coatings would impair the other desired characteristics of the ferrous material core.
When such thermal sprayed layers are used on gray cast iron brake disks, however, it was now surprisingly recognized that, because of the occurring metallurgical reaction with the material of the brake disk as well as the solidification course of the sprayed layer influenced by the high heat storing capacity of the brake disk, these thermal sprayed layers affect not only the wear behavior of the brake disk in a surprisingly positive manner even at high temperatures, but also reduce the heat crack formation and the braking torque fluctuations.
Alternative manufacturing methods to the preferred and proven arc welding are flame spraying, plasma spraying and explosion coating.
The invention therefore creates a protective layer for brake disks made of cast iron materials, which is applied to the friction surfaces of the brake disk by thermal spraying, in which case the sprayed-on material is a metal. In particular, the sprayed-on material is selected such that, in addition to the protection against wear, a protection against oxidation and against undesired metallurgical changes of the friction surface is achieved.
The injection molded coating preferably consists of a metal which is harder than the core, particularly unalloyed or alloyed steel. The injection molded coating preferably consists of a steel alloy containing Cr. In particular, the Cr content of the coating is between 10 and 20%, between 13 and 19%, between 15 and 17% or particularly preferably at 16%.
A brake disk has been particularly successful in the case of which the alloyed steel forming the coating contains the following additions: 16% Cr, 0.44% Ni, 0.43% Mn, 0.01% Mo, and 0.36% C.
It also proved to be useful that the core consists of a cost-effective gray cast iron. The gray cast iron forming the core is preferably a highly carburized gray cast iron which has a carbon content of from 3.7 to 4.0% and contains Mo and/or Cr. In particular, the Mo content of the core is at 6 to 8% and the Cr content is at less than 5%.
Axial coating thicknesses of from 0.3 to 1.5 mm, preferably 0.5 to 1.2 mm, particularly 0.6 to 0.9 mm, have also been successful. By means of a coating thickness of 0.7 mm, using a injection molded coating with a high Cr content, a doubling of the service life of the brake disk can often be achieved.
The invention is preferably used in the case of commercial vehicle brake disks with a disk thickness of from 38 to 45 mm and an air channel width of 12 mm.
The ratio between the disk thickness D1 and the air channel width D preferably meets the condition 3.2<=D1/D2<=4.
Reciprocally engaging contours, preferably a groove structure and/or a knob structure, are preferably constructed in the transition area between the core and the injection molded coating.
The friction rings expediently have at least one axial collar on the inner and/or outer circumferential edge, the height of this collar corresponding preferably to the axial dimension of the coating.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.